QUANTIFYING WATER EXCHANGE BETWEEN THE BRAZOS RIVER AND THE BRAZOS RIVER ALLUVIAL AQUIFER USING HIGH TEMPORAL RESOLUTION MEASUREMENTS
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abstract
Texas had unmet water supply needs in 2010. These unmet needs are expected to increase through 2040, making accurate water budgeting increasingly more important. Current water budgets use simple assumptions to estimate groundwater discharges (Qgw) to rivers based on established, but limited, hydrograph separation techniques. In this study we compare Qgw estimates obtained with more direct high-frequency techniques, including specific conductance mass balances, differential gaging, endmember-mixing analyses (EMMA), and the Dupuit equation using the observed slope of the unconfined water table, to Qgw estimates obtained using hydrograph separation. Together, these methods provide updated, seasonal estimates of the groundwater discharge component of the water budget for the second longest river in Texas. Two contiguous stretches of the river were investigated. The first, upstream river stretch primarily gained groundwater and saw strong agreement in Qgw estimated with specific conductance mass balance, differential gaging, EMMA, and hydrograph separation. Qgw ranged from -18 to 50 m^3 /s, and correlated positively with river discharge. Negative and positive Qgw indicates net losing and gaining conditions, respectively. The second, downstream river stretch both gained groundwater and lost river water to the aquifer, again with strong agreement in estimated Qgw using the same four methods as above. Calculated Qgw ranged from -155 to 112 m^3 /s, and, in contrast to the upstream stretch, correlated negatively to river discharge. The Dupuit equation-based estimates differed significantly from the other methods estimates in both studied stretches of the river. It predicted a consistent, negative correlation between Qgw and river stage since the contrast in hydraulic heads between the aquifer and the river drives flow. Interestingly, just the opposite was observed in the upstream river stretch. Where and when the Brazos River is losing and gaining, and how much water is exchanged has implications for managers of the river and the Brazos River Alluvial Aquifer (BRAA) that surrounds it. The results of this study will help managers of these two water bodies better understand the dynamics of their connection, and make better decisions about how much water can be allotted to prospective users and when.
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GSA Annual Meeting in Denver, Colorado, USA - 2016
